Lithography is widely used in the manufacture of integrated circuits. In lithography, and more particularly photolithography, a photoresist layer is formed on a semiconductor substrate. The photoresist is then exposed in certain areas to actinic radiation. The areas that are irradiated are defined by a mask which is projected onto the photoresist by a lens system. The mask contains a pattern of transparent and opaque areas. The mask is exposed to actinic radiation, such as ultraviolet light (UV), which is transmitted through the transparent areas of the mask to cause a chemical reaction in corresponding regions of the photoresist.
In a negative type photoresist the radiation impacted areas of the photoresist become insoluble in a developing solvent. For example, the radiation can initiate cross-linking, chain growth, photocondensation, or other such reaction to cause a chemical change in the photoresist. In a positive type photoresist the irradiated areas become more soluble in a developing solvent. For example, the radiation can cause photodegradation of the photoresist molecular structure.
Advancements in photoresist materials and methods have played a key role in the miniaturization of integrated circuits. Chemically amplified resists are an important class of photoresists for imaging wavelengths at or below 248 nm. Chemically amplified resists typically include four components: a base polymer with protected chemically reactive, hydrophobic groups; a photoacid generator (PAG); a base; and a solvent. Upon exposure to UV or other type of actinic, or activating, radiation, the PAG photodecomposes and generates a proton, H+. During a later post-exposure bake (PEB), the H+ acts as a catalyst to convert the hydrophobic protected groups on the base polymer into strong hydrophilic groups such as —COOH. This conversion, which is often called deprotection, makes a positive resist soluble in the developer.
One problem encountered with chemically amplified resists is the presence of water in the post exposure resist. The high affinity of the photo-generated proton for water severely interferes with its catalytic performance. Spatial variations in water concentration therefore causes uneven development, which in turn causes line broadening, blobs, and other defects in the completed integrated circuit.
In light of such problems, there is a need for improved methods and materials in high-resolution lithography.